MODES OF MAMMALIAN EVOLUTION 663 



The same conception was adopted and elaborated by Neu- 

 mayr: "Still other characteristics appear, which mark muta- 

 tions as something different from varieties, especially that, as 

 a rule, there is a definite direction of mutation in each series, 

 the same characters changing in the same sense through a con- 

 siderable succession of strata." ^ 



Whether development was continuous or discontinuous, 

 there is no reason to suppose that the amount and rate of modi- 

 fication were always constant. On the contrary, there is 

 strong evidence that at times of great climatic or geographical 

 changes, or when a region was invaded by a horde of immi- 

 grants, widespread readjustments were accomplished with 

 comparative rapidity. Indeed, such periods of relatively quick 

 changes have long seemed to be implied by the facts of the 

 palseontological records. 



It is only too clear that the principles as to the modes of 

 mammalian development which can be deduced from the his- 

 tory of the various groups must, for the most part, be stated 

 in a cautious and tentative manner, so as not to give an undue 

 appearance of certainty to preliminary conclusions, which 

 should be held as subject to revision with the advance of know- 

 ledge. Much has, however, been already learned, and there is 

 every reason to hope that Experimental Zoology and Palaeon- 

 tology, by combining their resources, will eventually shed full 

 light upon a subject of such exceptional difficulty. 



iM. Neumayr, Die Stamme des Thierreiches, Bd. I, p. 60. 



